A flow battery has become one of ideal choices of large-scale energy storage owing to its advantages, such as longevity, high safety, powerful over-charging and over-discharging capacities and environmental friendliness, and mainly functions in application markets including renewable energy source power stations, user side smart microgrids (in residential areas, industrial areas and communal facilities), etc. Correspondingly, the flow battery system has multiple functions, i.e., utilizing valley electricity in peaks, balancing loads, improving the quality of electric energy, etc.
A state of charge (SOC) is a parameter reflecting the status of electric energy stored in a battery, is the most direct basis for a battery system to realize accurate control and management, and is also one of the most important parameters of the flow battery. The real-time and accurate SOC plays critical roles in guaranteeing high reliability operation of the battery system, improving the service effect of the battery and prolonging the life of the battery. In order to ensure effective control and management of the flow battery and achievement of favorable charging and discharging performances and long service life, it is necessary to detect SOC of the flow battery and record real and accurate SOC values at all times, and further control the flow battery to execute corresponding operating strategies according to the values, e.g., adjusting electrolyte flow, changing charging and discharging modes, etc. Furthermore, a battery management system also further feeds back the obtained SOC values to a overall energy management system to provide important references and basis for scheduling thereof. That is to say, the degree of accuracy (i.e., the deviation with a real value) of the SOC values obtained by SOC detection devices will directly affect the operating safety and stability of the flow battery, even a power station-level energy storage system.
State of charge (SOC) of the flow battery is learnt mainly by monitoring a real-time voltage difference between anode electrolyte and cathode electrolyte. The voltage difference is directly related to electrolyte concentration. In a working flow battery system, electrolyte in an anode electrolyte storage tank and electrolyte in a cathode electrolyte storage tank flow through an electrolyte circulating pipeline and the stack driven by a circulating pump, and undergo electrochemical reactions in the stack, such that the concentration of active components in the electrolytes entering the stack changes; then, the electrolytes return to the anode electrolyte storage tank and the cathode electrolyte storage tank and are mixed with electrolytes in the storage tanks, and therefore the voltage differences between the anode electrolyte and the cathode electrolyte of the flow battery in different locations are different, and accordingly, the voltage differences of electrolytes at any position of the flow battery, e.g., an stack inlet, a stack outlet as well as in the anode electrolyte storage tank and the cathode electrolyte storage tank cannot directly reflect real-time state of charge (SOC) of the flow battery; in general, state of charge (SOC) of the flow battery system in the prior art reflects the state of charge (SOC) of the whole flow battery system merely by monitoring a voltage difference between anode electrolyte and cathode electrolyte of the flow battery in a single position. Other numerous factors, such as power/capacity configuration and charging and discharging stages have not been taken into consideration yet. Such measurements are conducted according to unified standards, and real-time and accurate state of charge (SOC) cannot be monitored and calculated comprehensively and completely.
In addition, only one set of SOC detection devices is placed at a monitoring point in the prior art, which means that a battery management system cannot judge whether the detected SOC value is an accurate valve because of lack of reference data for comparison, when the SOC detection device is damaged, and it is possible to cause serious affects on the operating safety and stability of a flow battery body and a power station-level energy storage system in case of controlling and scheduling the flow battery based on inaccurate SOC value. Specifically, in cases of electrolyte leakage and blockage, the imprecisions of a potential sensor or a voltage sensor in a monitoring position in the SOC detection devices, it is possible to cause a greater deviation between the SOC fed back between the SOC detection device and the real value. It is known based on information gathered from multiple megawatt-level flow battery projects domestically and abroad at present that the differences between the SOC value measured by the SOC detection device to the battery management system and the real SOC value even exceeds 10% in some cases. When the SOC detection device feeds back the inaccurate SOC value to the battery management system and a high-level energy management system, the inaccurate SOC value at least would affect subsequent operation and management of the flow battery, and in more serious cases will cause the situation that a scheduling instruction significantly fails to conform to the real state of the flow battery, thus causing forceful over-charging and over-discharging of the flow battery and seriously affecting the operating efficiency and stability of the whole energy storage system, and the phenomena, such as significant reduction of capacity and performance of the battery system, stack burnout and failure to continuous work of the battery system will occur if this case continues in this way.
In actual operations, an electrical power system or a high-level scheduling system pays closer attention to actual chargeable and dischargeable capacities of the battery system. When some operating parameters of the flow battery, such as temperature, operating mode, electrolyte flow and electrolyte temperature are off specifications, SOC obtained by the SOC detection device does not directly reflect the electric quantity actually dischargeable from the flow battery, and if SOC is simply used for reflecting chargeable and dischargeable capacities, it would cause inaccurate scheduling and over-charging/over-discharging of the flow battery, or misjudgment of the scheduling system and the like, thereby seriously affecting the operating efficiency and stability of the whole energy storage system and a power station.
Furthermore, the input-output characteristic of an alternating-current side of a flow battery is one of the problems that people concern, and is a premise that a user can use the flow battery favorably and accurately. But, the flow battery itself has a magnetic drive pump, a heat exchange system, a ventilation system, a battery management system, a sensor and other auxiliary power consumptions, and for the flow battery, when it is charged or discharged, it is necessary to provide extra power consumption and energy to drive the auxiliary power consumptions to run, such that the input-output characteristic of the alternating-current side of the flow battery is significantly different from that of the traditional storage battery; secondly, self-discharging of the flow battery is also significantly different from that of the traditional battery, and is less affected by time, but greatly affected by a capacity-to-power ratio; finally, the same as the conventional storage battery, the flow battery also refers to the alternating-current/direct-current conversion efficiency of an energy storage inverter and a transformer. The above factors determines that the input-output characteristic of the alternating-current side of the flow battery cannot be estimated accurately, e.g., under the current system state and under different operating modes, what is the maximum power that the alternating-current side can bear and what is the maximum energy capable of being charged or discharged by the alternating-current side are often more concerned by users.